Low-frequency amplifier

ABSTRACT

The invention relates to a low-frequency amplifier with an integrated push-pull B final stage and a circuit for adjusting the quiescent current. The invention resides in the amplifier comprising two circuits for adjusting the quiescent current which are provided for different voltage ranges of the supply voltage. An electronic switchover device ensures automatic activation of that circuit for quiescent current adjustment which is suited for the respective voltage range.

BACKGROUND OF THE INVENTION

The invention relates to a low-frequency amplifier with an integratedpush-pull class B final stage and a circuit for adjusting the quiescentcurrent.

The permissible operating voltage range required for integratedlow-frequency push-pull class B amplifiers is becoming progressivelylarger. The reason for this is that these low-frequency amplifiers areto be used for all kinds of equipment, for example, for portable setswith headphones as well as for medium-sized and larger equipment, withpower supply overvoltages being taken into consideration, requiresoperating voltages of up to 16 V.

A further requirement exists with respect to adjustment of the quiescentcurrent. As small a quiescent current as possible is desired in order tokeep the load on batteries low. Crossover distortions can, however,occur with quiescent currents which are too low and, therefore, acompromise must also be sought with respect to adjustment of thequiescent current. The output dc voltage of the low frequency push-pullclass B amplifier should have the optimum value required for maximumlow-frequency output power. The output dc voltage should, therefore,correspond approximately to half of the operating voltage.

Two suitable low-frequency amplifier circuits which meet some of theabove-mentioned requirements are known from German Patent No. 3,409,417.These known circuits are shown in FIGS. 1 and 2 and, for betterunderstanding, are explained in detail hereinbelow.

The final stage shown in FIG. 1 comprises transistors T12 and T13 of thesame conduction type. The lower final stage branch with transistor T13is driven via a complementary transistor T11 which simultaneouslyeffects the phase reversal. The base of this complementary transistorT11 is connected to the collector of driver transistor T1. The collectorof the driver transistor T1 is connected via diode D2 and current sourceQ3 to supply voltage U_(s). The voltage at point M constituting theoutput of the final stage is adjusted to half of the battery voltage bya control circuit which is not illustrated. This point M is connected tothe junction between the final stage transistors T12 and T13 andconstitutes the output of the amplifier circuit with the load R_(L).

In the circuit shown in FIG. 1, the adjusting circuit for the quiescentcurrent comprises diode D2, across which a voltage drop U2 is generated,and diodes D7 and D8 across which corresponding voltage drops U7 and U8are generated. The anode connection of the diode series circuitcomprising diodes D7 and D8 is at the base electrode of transistor T10,complementary with transistor T11, and is further connected via acurrent source Q9 to the positive pole of supply voltage source U_(s).The cathode of diode D8 is at point M and hence at the output of thefinal stage amplifier. The emitter electrodes of transistors T10 and T11are interconnected. The collector of transistor T10 is connected to thepositive pole of the supply voltage source. The junction point betweensource Q3 and diode D2 is connected to the base electrode of atransistor T4 whose emitter is at M and whose collector branch containsdiode D5, across which the base-emitter voltage for transistor T6 drops.The collector of transistor T6 is connected to the control or baseelectrode of the final stage transistor T12 and can be connected to Mvia a resistor R23. Diodes D2, D7 and D8 together with the base emitterpaths of transistors T4, T10 and T11, form a voltage loop. The sum ofthe voltages at the base-emitter paths of these transistors consequentlycoincides with the sum of the voltages at diodes D2, D7 and D8. Thevoltage drop across diodes D2, D7 and D8 can be adjusted bycorresponding selection of the currents of current sources Q3 and Q9such that the desired current flows in transistor T4 and in transistorT11, respectively, which then predetermines the quiescent current offinal stage transistors T12 and T13. On account of the necessary voltagedrop across diodes D7 and D8, the supply voltage in the circuit of FIG.1 must not drop below 3 V as the voltage drop required for operatingcurrent source Q9 would otherwise not be large enough. On the otherhand, this disadvantage is contrasted with the advantage that thecurrent flowing through diodes D7 and D8 is very small since the basecurrent of transistor T10 is smaller by this current gain factor thanthe current flowing through collector T11.

In the circuit shown in FIG. 2, the supply voltage can be furtherreduced. In this circuit, transistor T10 is eliminated and the diodechain consisting of diodes D7 and D8 is replaced by a single diode D14.The anode of this diode D14 is directly connected to the emitter of thecomplementary transistor T11, whose collector drives the base electrodeof final stage transistor T13. The voltage at point M is, therefore,only raised by a diode forward ,voltage U14. Thus the potential at theanode of diode D14 is approximately 0.7 V lower than the comparablepotential at the base electrode of transistor T10 in FIG. 1. The voltageloop is formed by diodes D2 and D14 and by the base-emitter paths oftransistors T4 and T11. Therefore, the voltage U_(s) can drop to valuesof up to 1.8 V as a voltage of approximately 1.6 V is then present atthe emitter of transistor T11 and the residual voltage of 0.2 V is justadequate to operate the current source Q15. When the circuit is drivenby a signal, a current which increases as the signal increases flowsthrough final stage transistors T12 and T13. Hence the base current oftransistor T13 also rises and, consequently, the emitter current oftransistor T11. To enable transistor T11 to prepare for this change inthe emitter current, the current flowing through current source Q15 andhence also through diode D14 must be chosen relatively large incomparison to the emitter current of transistor T11. Therefore, in thecircuit of FIG. 2 the advantage of a reducible supply voltage iscontrasted with the disadvantage of the higher current consumption.

SUMMARY OF THE INVENTION

The object underlying the invention is to provide a new circuit for alow-frequency amplifier which combines the advantages of both knowncircuits in a suitable way. To achieve this object, in a circuit of thekind described at the outset, it is suggested, in accordance with theinvention, that two different adjusting circuits be provided for thequiescent current, with the first quiescent current circuit (FIG. 2)being optimized with respect to the voltage drop and the secondquiescent current circuit (FIG. 1) being optimized with respect to thecurrent consumption, and that an electronic switchover device beprovided to activate at low supply voltage the first quiescent currentcircuit, with the latter being switched off when a certain value of thesupply voltage is reached and adjustment of the quiescent current thenbeing effected solely via the second quiescent current circuit.

A differential amplifier is preferably provided as switchover device,with a stabilized dc voltage at one input electrode and the other inputconnected to the tap of a voltage divider connected to the supplyvoltage. The voltage divider is so dimensioned and the stabilized dcvoltage so selected that at low supply voltage the first quiescentcurrent (FIG. 2) circuit is activated via a current mirror circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a conventional low-frequencyamplifier having a relatively low quiescent current;

FIG. 2 is a schematic diagram illustrating a conventional low-frequencyamplifier which will operate at a lower supply voltage than theamplifier of FIG. 1 but which has a higher current consumption; and

FIG. 3 is a schematic diagram illustrating a low-frequency amplifieraccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 shows a circuit of the inventive low-frequency amplifier in whichthe circuit elements have been designated in the same way as thecorresponding circuit elements in FIGS. 1 and 2. The circuit of FIG. 3contains two quiescent current circuits, with the first quiescentcurrent circuit corresponding to the circuit described in connectionwith FIG. 2. The second quiescent current circuit (FIG. 1), which isoptimized with respect to the current consumption, corresponds to thecircuit described in conjunction with FIG. 1. However, the circuit ofFIG. 3 contains, in addition, a differential amplifier comprising thetwo transistors T17 and T18 connected at the emitter electrodes, withthese emitters being connected to reference potential via resistor R21.Located in the collector branch of transistor T17 is a diode D16,conductive in the flow direction, which is arranged parallel to thebase-emitter path of transistor T15 and forms with it current source Q15(FIG. 2). Transistor T15 supplies the current for diode D14, whichserves in the above-described manner to adjust the quiescent current. Astabilized dc voltage of, for example, 1.2 V is present at the baseelectrode of transistor T17. The base voltage of transistor T18 is takenfrom a voltage divider comprising resistors R19 and R20 at its tap. Theabove-mentioned voltage divider is connected to supply voltage U_(s) andthe base potential of transistor T18 is, therefore, directly dependenton the supply voltage. The collector of transistor T18 is connected tothe positive pole of the supply voltage source.

The switchover device is so dimensioned that at low voltage values ofthe dc supply voltage U_(s), the base potential of transistor T18 is notadequate to drive it as the stabilized dc voltage U_(stab) at the baseelectrode of transistor T17 is larger. Consequently, transistor T17 isconductive and hence the current mirror comprising diode D16 andtransistor T15 is also activated. Since the anode of diode D14 isconnected to the emitter of the complementary transistor T11, theadjustment of the quiescent current is carried out in the mannerdescribed in conjunction with FIG. 2.

At larger values of the supply voltage U_(s), the voltage at the baseelectrode of transistor T18 exceeds the stabilized dc voltage at thebase electrode of transistor T17 and, therefore, this transistor T17 isblocked. Hence diode D14 becomes currentless. Quiescent currentadjustment is now controlled in accordance with the arrangement of FIG.1 by diodes D2, D7, and D8 and transistors T4, T10 and T11. Diodes D7and D8 are driven via current source Q9, with a relatively low currentbeing adequate since current amplification occurs in transistor T10. Ina suitable embodiment of the circuit, the dimensioning is such that thecurrent source comprising diode D14 is switched off at a supply voltageof from 2.5 V to 3 V. At smaller voltage values, the current source Q9remains inactive since the supply voltage is inadequate to supply thenecessary voltage drops across diodes D7 and D8.

The inventive circuit offers the great advantage that with an embodimentof a low-frequency amplifier supply voltages can be tolerated in a verylarge range of, for example, between 1.6 V and 16 V. In the circuit ofFIG. 3, transistors T17 and T18 of the switchover device are preferablynpn transistors. T15, T11 and T6 are pnp transistors.

What is claimed is:
 1. A low-frequency amplifier for use with a powersource which provides a supply voltage, comprising:means for receiving asignal to be amplified; a quasi-complimentary final stage which receivespower from the power source; a complimentary transistor, connectedbetween the final stage and the means for receiving a signal to beamplified, to effect phase reversal; and means for setting a quiescentcurrent through the final stage, the means for setting includinga firstquiescent current circuit which is optimized for low-voltage use, thefirst quiescent current circuit being selectively actuatable, means foractuating the first quiescent current circuit when the supply voltage isbelow a predetermined valve, and a second quiescent current circuitwhich is optimized for low current consumption, the second quiescentcurrent circuit setting the quiescent current at least when the firstquiescent current circuit is not actuated.
 2. The low-frequencyamplifier of claim 1, wherein the means for actuating the firstquiescent current circuit comprises: a voltage divider connected to thepower source, the voltage divider having a tap; a differential amplifierhaving an input port which is connected to the tap of the voltagedivider and having another input port which receives a stabilized dcvoltage, the differential amplifier additionally having an output port;and a current mirror circuit connected to the output port of thedifferential amplifier, wherein the voltage divider and the stabilizeddc voltage are selected so that the first quiescent current circuit isactuated via the current mirror circuit when the supply voltage is belowthe predetermined valve.
 3. The low-frequency amplifier of claim 2,wherein the current mirror comprises a current-source transistor, andwherein the first quiescent current circuit comprises avoltage-generating diode which is connected to the current-sourcetransistor and to the complimentary transistor, the value of the currentflowing through the voltage-generating diode being selected so that,when the final stage is driven at full power, the diode current isadequate to maintain the forward voltage of the diode.
 4. Thelow-frequency amplifier of claim 3, wherein the final stage has anoutput port, wherein the cathode of the diode is connected to the outputport, and wherein the anode of the diode is connected to the emitter ofthe complimentary transistor.
 5. The low-frequency amplifier of claim 1,wherein the amplifier comprises a further transistor, the emitters ofthe further transistor and the complimentary transistor being connected,and wherein the second quiescent current circuit comprises twoseries-connected diodes, the base of the further transistor being drivenby a dc voltage drop at the pair of diodes.
 6. A low-frequency amplifierfor use with a power source which provides a supply voltage,comprising:a transistorized push-pull class B final stage which receivespower from the power source; means for setting the quiescent currentthrough the final stage at a predetermined small value of the meansemploying circuitry which requires a supply voltage of more than apredetermined voltage value for operation; further means for setting thequiescent current through the final stage at another value which islarger than the predetermined small value, the further means beingselectively actuatable and employing circuitry which does not require asupply voltage of more than the predetermined voltage value foroperation; and switchover means, responsive to the supply voltage, foractuating the further means when the supply voltage is too low foroperation of the means.
 7. The low-frequency amplifier of claim 6,wherein the means for setting the quiescent current through the finalstage at a predetermined small value comprises first and second diodesconnected in series, one of the first and second diodes being connectedto the final stage, and wherein the further means comprises a furtherdiode which is connected to the final stage and to one of the first andsecond diodes.
 8. The low-frequency amplifier of claim 7, wherein thefurther means further comprises a further transistor which is connectedbetween the power source and the further diode, and wherein theswitchover means comprises means connected to the base of the furthertransistor for comparing the supply voltage with a reference voltage. 9.The low-frequency amplifier of claim 6, wherein the power source hasfirst and second supply terminals, wherein the final stage comprisesfirst and second transistors, the collector of the first transistorbeing connected to the first supply terminal, the emitter of the secondtransistor being connected to the second supply terminal, and theemitter of the first transistor being connected to the collector of thesecond transistor, wherein the means for setting the quiescent currentthrough the final stage at a predetermined small value comprises firstand second diodes, the cathode of the first diode being connected to theanode of the second diode and the cathode of the second diode beingconnected to the emitter of the first transistor, and wherein thefurther means comprises a third diode, the cathode of the third diodebeing connected to the emitter of the first transistor.
 10. Thelow-frequency amplifier of claim 9, wherein the further means furthercomprises a third transistor, the emitter of the third transistor beingconnected to the first supply terminal and the collector of the furthertransistor being connected to the anode of the third diode, and whereinthe switchover means comprises a voltage divider connected to the firstsupply terminal, a differential amplifier having an input port which isconnected to the voltage divider and having another input port whichreceives a reference voltage, the differential amplifier additionallyhaving an output port which is connected to the base of the thirdtransistor.
 11. The low-frequency amplifier of claim 6, wherein thepredetermined voltage value is about three volts.